Typically, methods for acquiring decoded signals by removing various types of interference including interference due to multi-path fading, inter-symbol interference, and multiple access interference and the like, adopt joint detection decoding.
Joint detection decoding is an example of a decoding method that utilizes matrix operation. That is, in joint detection decoding, predetermined matrix operation is executed using a system matrix in which convolutional operation results of the channel estimation values of individual communication terminals (users) and the spread codes that are assigned to the communication terminals (users) are provided in matrix, and the matrix operation results are multiplied by a received signal's data portion so that interference is cancelled and decoded data is obtained. Thus joint detection decoding such as above obtains decoded data wherefrom interference is cancelled. Compared to RAKE combining, joint detection decoding is characterized by high performance of interference cancellation and high reliability of decoded data.
However, with conventional joint detection apparatus, when a signal (i.e. code, frame) of a high received level and a signal (i.e. code, frame) of a low received level are subjected to joint detection decoding, their signal levels become substantially the same as a result. These signal levels do not reflect the signals' likelihood, and when an error correction decoding scheme is employed that uses the scale of decoding result for likelihood information, the accuracy of error correction does not improve.